Development of Sorption Cooler for 1 K Adiabatic Demagnetization Refrigerator

Abstract

The sorption cooler, which utilizes helium-4 as a working fluid, is developed for virtual precooling purpose of the 1 K ADR (Adiabatic Demagnetization Refrigerator) and tested in this study. The system consists of an adsorption pump, a thermosyphon and a tin strap. The cooler rejects heat to a 4.5 K heat sink, which is made by a commercial 2-stage Gifford-McMahon cryocooler. The adsorption pump contains activated charcoal of 5.2 g, and thermally connected to the heat sink through a tin strap. Its temperature in controlled from 5 to 30 K by a heater installed on the pump. The thermosyphon is filled with various helium filling ratios while the pump maintains its temperature at 30 K. In this operational stage, the assembled precooling system just works as a passive thermosyphon. When the heater on the pump turns off and the temperature of the pump decreases, helium in the thermosyphon is spontaneously transferred to the pump, and the sorption cooling starts. The cooling capacity of the cooler is measured by a heater installed on the evaporator of the thermosyphon. In a typical experimental condition, the temperature of the evaporator drops under 2.17 K, and the production of the superfluid is confirmed. This cooler is planned to be assembled with the 1 K ADR, to constitute the cascade refrigeration system which can obtain sub-kelvin temperature (<1 K).